Process of regenerating activated carbon loaded with adsorbed gold



J. B. ZADRA PROCESS OF REGENERATING ACTIVATED March ll, T952 CARBONLOADED WITH ADSORBED GOLD Filed March 25, 1950 Il UIQ- JOHN @.ZARA

'v INVENTOR BY `ATTORNEY Patented Mar. 11, 1952 ,PROCESS F REGENERATINGACTIVATED CARBON ,LOADED WITH ADSQRBED YG QLD John-Benjamin Zadra, Reno,Nev.,.assignor-to the United-States, of America-as representedby theSecretary of ,the Interior `ApplicatiorrMarch 25,51950, "Serial'Nm151,999

1 Claim.

(Granted .under y,the act,of March 3, .1883, as

Vamended l April .30, 1928; 370 0. G. A,'757) The inventiondescribedherein may be manufactured and used -by -orfor the Government-of theVUnited States `for vgovernmental purposes withoutfthe paymentto me ofany royalty thereon in accordance -With -theprevisions of the act-ofApril 30, 1928 (ch. 460, 4j5fStat. VIii-167).

This invention relates to the production of gold and more particularlyto the regeneration of activated carbon employed for adsorbing orabsorbing gold from dilute.` cyanide solutions thereof.

In my co-ypending application Serial No.jl27',7.61 filed Novemberv 16,1949, now PatentNo, 2,579,531, entitled Process forlExtracting Gold andSilver, there has been described a process vfor regenerating activatedcarbon loaded with adsorbed gold involving treating such carbon .withcaustic sodium sulfidesolution to extract the gold therefrom, separating.the thus-regenerated activated carbon, stripping the gold Afrom thepregnant caustic Ysodium sulfide solution lby electrolysis between aninsoluble anode and a precoated charred excelsior cathode, andrecovering the .deposited gold. In this process it ,is necessary toprecoat the. charred excelsior cathodeby employing itV as the cathodeina gold cyanideelectrolyte so as to deposit an amount of gold thereonsuiii- .cientto reduce its internal resistance to an operative point.This precoating. constitutesan additional phase of the process which itwould be desirable to obviate. Additionally, charred excelsior is aparticularly fragile substance which can be handled ormanipulated-onlywith extreme care. It has A nowbeen found that by mea-nsof the'instant improvement one neednot precoat the cathodematerialbutinstead may use -a cathode material of great durability and which isreadily available.

In accordance with this invention, there is employed as a cathodematerial in my abovedescribed processstainless ,steel wool, `preferablyalstainless `steel Wool made from Astainless steel containing 4aboutY17% chromium and known to the trade as Type-130, Medium Grade. Otherstainless steels may, of course, be employed, the only .requisitebeing.that they be not substantially corroded by caustic sodium sulfidesolution. For example, stainlesssteelscomposed of iron containing 18%chromium and V8% nickel and those in which a portion -of the iron isreplaced by cobalt can be employed; although, as above stated,itehasibeen found Atfha-ifa'l7% chromium stainless steel is not corrodedby the electrolyte but is readily disintegrated as will beeXplnedhereinafter.

As explained in my above-mentioned co-pending application, the-gold-bearingactivated carbon isl leached with caustic alkalisuldesolution, suitably one containing from 1 -to 5% `Stadium suliide andfrom 2 to 8% sodium hydroxide. .The thus-regenerated activated carbon iswashed and returned to the mill circuit for further adsorp- .vided iwitha'negative contact 2.

tion/.of `precious-metal values. The gold-bearing `causticsodium sulfidesolution is then employed Vas-anelectrolyte between an insoluble anodeand `the stainless steel -wool cathode of this invention.

It is unnecessary to precoat the cathode. Ingen- .,eral, a voltagebetween twov and six volts, preferablyqabout'three volts, has been foundsatisfactory in the operationof the electrolysis.

Any suitable electrolytic cell may be employed, such as, for example,the cell described in`-my above-mentionedapplication Serial No. 127,761co-pending herewith, led November 16, 1949. A very satisfactoryelectrolytic cell is the circular one described in my co-pendingapplication Serial No. 131,654 led December '7, 1949, now PatentNo.2,563,-903 dated August 14, 1951, entitled Electrolytic Cell for theDeposition of Gold and-Silver from Solutions.

`As shown-in the aforesaid application Serial No. 131,654, which wasallowed May l5, 1951, a suitable -electrolytic lcell, in which there canbe employed the stainless steel Vwool 'cathode of jthis invention,

is illustrated i-n the accompanying drawing. Referring to the drawing,the cathode assembly of the cell comprises, in part, a verticallymounted, 'electrically conductive inlet pipe :I for lintroducing goldsulfide or other metalbearing `solution-into the cell. The pipe I alsoserves as a negativebusbar or conductor of electricity -in-to the celland Kfor this purpose is pro- The pipe I is fastened at its lower endtoa radially extended disc 3 in such fashion 'as-to make an electricallyconductive joint, as Vby welding. Near the lower end of the pipe Iis anaperture 4 for permitting the Asolution Ventering .through the ypipe rIto emerge therefrom into contact with a surround- In order that betterelectrical contact vmay be made between pipe I and the` stainless steelwool 5, the disc 3 is provided with a plurality of vertically extendingWire rods 'E `spaced about-and parallel to the pipe I. 'The rodsIi, theplate 3 and the pipe I are Vconstructed:Ofian electrically conductivematerial which is Ynot substantially corroded rby the solutionpbeingelectrolyzed. In the case of a caustic alkaline solution of gold, -thesemembers can suitably be .constructed of "stainless-steel, preferably thestainless steel known as "18-8 which containsabout 18% chromium andabout 8% 'nickel` the balance being substantiallyall iron.

Inprder to restrain the stainless steelwool 5 in position, a retainingmember is provided which isof vsuch form-as to -permitthe egress ofsolutiongfrom contact with the cathode material into contact with acircumierentially Adisposed anode. As shown, the retainingimember maytake the .form of aperforated basket 'I made of hard rub- -ber or othernon-conductive material, into which `nests the plate 3 andwhich'restrainsthe*stainless steel wool-5. As shown,theperioratedfbasket I engages the plate 3 so as to space the rods 6 andthe pipe I in their correct locations. A suitable anode 8 peripherallyencircles the basket 1 and, as shown, such anode may take the form ofstainless steel wire mesh rolled to define an open cylinder. The meshanode 8 is provided with a positive terminal 9 and is spaced closelyadjacent to and encircling the retaining basket 1. The anode 8 and thecathode assembly above described are placed in a, concentricallysurrounding weir cup I which serves the purpose of controlling the fluidlevel in the electrolytic cell while, at the same time, restraining theanode 8 into an equally spaced relationship to the stainless steel woolin the cathode assembly. The weir cup I 0 is provided with cylindricalside walls and preferably a closed bottom, the whole being constructedof hard rubber or other suitable non-conductive, non-corrosive material.A launder II, provided with an outlet I2, embraces the Weir cup I0 andthe whole assembly to provide a supporting base therefor. The launder IIcollects the overow from the Weir cup I0 and directs the strippedsolution away from the cell through the outlet I2.

In operation, the retaining basket 'I and the cooperating cathodeconducting assembly defined by the pipe I, plate 3 and the rod 6 arefilled with stainless steel wool 5. Thereupon, a caustic sodium sulfidesolution of gold or other metal-bearing solution is introduced throughthe pipe I and proceeds through the aperture 4 into contact with thestainless steel wool 5 and the anode 8. After a sufficient quantity ofsolution lls the Weir cup I0, it overflows into the launder I I andpasses out through the outlet I2. The contact '2 is connected with asource of negative current not shown, and the anode connection 9 issimilarly connected to the positive current source. Electrolysis thenproceeds and gold or other metal to be deposited is deposited largely onthe stainless steel wool 5. The electric current passes by way of thesolution through the perforations in the basket 'I to the anode 8 andthence out of the cell through the positive terminal 9. As soon as thegold or other metal content of the initial solution has been strippedtherefrom, additional gold-bearing solution may continuously beintroduced through the pipe I and continuously stripped of its metalcontent.

Stripped solution is withdrawn from the cell through the outlet I2 andmay be used for the solution of additional quantities of gold or thelike. After the stainless steel wool 5 has taken up a sufficientquantity of gold or other metal, passage of current and solutionthroughthe cell is discontinued and the retaining basket 'I, together with thestainless steel wool 5 and the pipe I, are withdrawn from the cell. Theplate 3 is withdrawn from the basket 'I by means of the attached pipe Iand the loaded stainless steel wool 5 discharged for working up of itsgold content. Thereupon, the plate 3 and attached rods 6 and pipe I arereinserted in position into the basket and recharged with additionalstainless steel wool 5. The basket and enclosed cathode assembly arethen reinserted into the weir cup I0 inside the anode 8. The cycle canthen be repeated.

The following illustrative example shows how this invention may becarried out but it is not limited thereto:

Example 38 grams of stainless steel wool containing 17% chromium and thebalance being substantially all iron (Type 430) was placed in thecathode compartment of an electrolytic cell provided with means forintroducing electrolyte into the mid-portion of the stainless steel woolcathode, and contact means for connecting the negative bus bar to thestainless steel wool. A stainless steel screen anode was disposed aboutthe periphery of the stainless steel wool cathode but was prevented fromcontacting the same by means of a hard rubber separator of circularcross-section. Four liters of the gold impregnated stock sodium suldesolution was circulated through the cell at the rate of 4 liters perminute while a current of 9 amperes was used. The electrolyte wasemployed at a temperature between room temperature and about 35 C. and avoltage of between 2 and 5 volts was required to secure the desiredcurrent. The test was repeated four times over a period of 7 daysreusing the same stainless steel wool in order to determine the effectof rust or local corrosive action if any. The results of the tests aretabulated as follows:

Milllgrams of gold per liter From the tabulated data'l it is apparentthat the extraction efficiency improved with each succeeding test. Atthe completion of the tests the gold-laden steel wool was dissolved inhot 25% sulfuric acid to dissolve out as much iron as possible from thestainless steel wool. The remaining residue was washed with water, driedand then melted in a graphite crucible while fluxing it with soda ash,borax, silica and nitre. A high purity gold bullion assaying 996 fineand a clean slag were obtained.

Since many apparently differing embodiments of this invention will occurto one skilled in the art, various changes may be made in theembodiments herein described without departing from the spirit and scopeof the invention.

What is claimed is:

In a process for regenerating activated carbon loaded with adsorbed goldinvolving dissolving the gold content with caustic sodium sulfide,separating the regenerated active carbon, and electrolytically strippingthe dissolved gold between an insoluble anode and a cathode, theimprovement which comprises employing stainless steel wool as thecathode and recovering the gold therefrom, after deposition thereon, bydissolving said stainless steel wool in hot sulfuric acid and separatingthe solution from the precipitated gold.

JOHN BENJAMIN ZADRA.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 591,571 Richards oct. 12, 18972,325,660 Chamberlain Aug. 3, 1943 OTHER REFERENCES Transactions of theAmerican Electrochemical Society, vol. 35, published 1920, pages 251,253, 25'7.

